tests: Screenshot is not available until vm is started

Fix the VM lifecycle test.  The test exploited an old bug in the Mock
Model which allowed a screenshot to be viewed when the VM is powered
off.  Now that the bug was fixed we need to move the screenshot check
later in the test.

Signed-off-by: Adam Litke <agl@linux.vnet.ibm.com>
Message-id: 1372436337-17341-1-git-send-email-agl@linux.vnet.ibm.com